1. Field of the Invention
The present invention relates to a small refractive zoom lens optical system applicable to a small mobile apparatus including a mobile optical system.
2. Description of the Related Art
In general, high-resolution refractive zoom lens optical systems are classified into the following two types. One has high resolution, but has a complicated structure and a large size, and the other has a small size and a simple structure, but has high resolution only at a specific distance to an object.
Such a refractive zoom lens optical system is disclosed in, for example, JP-A-2004-334070 or U.S. Pat. No. 7,085,070. The disclosed two systems have magnification that is about three times higher than that of a known refractive zoom lens. In addition, the disclosed two systems have a small size and a simplified lens structure.
However, the disclosed two systems have the following some technical problems.
First, the zoom lens optical system has a large size. When TTL is the total optical length and fw is a focal length when the optical system is at the widest angle, the disclosed two optical systems have a value of TTL/fw in the range of about 10 to 12. In recent years, however, a small size having the value of TTL/fw in the range of about 6 to 7.5 has been technically required.
Second, uniform resolution is not obtained. That is, high resolution is obtained at a specific distance to an object, but the resolution is lowered when auto focusing is performed from a short distance to a long distance.